1. Field of the Invention
This invention relates to processes for forming aqueous dispersions of photographic spectral sensitizing dyes and more particularly to processes for directly dispersing substantially water-insoluble cyanine dyes directly in water to form solvent-free compositions useful in the spectral sensitization of photographic silver halide emulsions.
2. Description of the Prior Art
It is well known in the photographic art that most spectral sensitizing dyes for photosensitive materials are highly insoluble in water, yet are most advantageously incorporated in aqueous systems, such as dispersions of the photosensitive material in a hydrophilic binder such as gelatin. The majority of these spectral sensitizing dyes have thus been dissolved in a variety of organic solvents, notably the lower alcohols such as methanol and ethanol, before incorporation into the aqueous system; however, the use of these solvents has long been considered undesirable for a variety of reasons.
The relative insolubility of most of the spectral sensitizing dyes even in the organic solvents indicated above has required that relatively large quantities of solvent and/or elevated temperatures be employed in preparing sensitizing dye solutions possessing concentrations and solution stabilities suitable for commercial application. An aqueous colloidal system, such as a photographic gelatino silver halide emulsion, may be deleteriously affected by the addition of such solvents, for example by causing a local dehydration or coacervation of the binder material resulting in particulate matter which cannot be readily redispersed. The resultant effects are colloidal instability and coating defects when the colloidal system is coated on a support. Furthermore, the hazards of fire and explosion associated with the use of large quantities of flammable organic solvents of the type indicated cause considerable concern in the art, not only because of the danger involved in their use, but also because of the manufacturing facilities required to store and dispense them.
Aqueous spectral sensitizing dye compositions devoid of organic solvents are known to the art. Most notably, Owens et al. U.S. Pat. No. 3,660,101 discloses and claims substantially solvent-free dispersions of a spectral sensitizing dye in a hydrophilic colloid. However, prior art methods for preparing such dispersions require that the dye first be dissolved in an organic solvent to form a highly concentrated dye solution, which is subsequently added to the hydrophilic colloid and the solvent removed in some manner such as volatilization. Similar processes are described, for example, in Thomas U.S. Pat. No. 2,360,289 and Roth et al. U.S. Pat. No. 2,991,177.
It would, of course, be considered quite desirable if organic solvents could be eliminated completely, not only from the compositions used to spectrally sensitize the photographic emulsions, but from all aspects of the spectral sensitization procedure. It is to this end that the present invention is directed.